In one known rear frame structure, a reinforcing member is provided to the rear frame of a vehicle, as is disclosed in JP 2005-119358 A. The reinforcing member is designed to cope with an impact load when an impact load (impact energy) acts on the rear part of a vehicle body in an offset collision when the vehicle is traveling at low speeds.
The rear frame structure in JP 2005-119358 A includes left and right rear frames extending along the longitudinal direction of the vehicle body, a cross member extending between the left and right rear frames, and a rear end panel spanning between the rear ends of the left and right rear frames.
Left and right diagonal members extend in a straight line from regions where the left and right rear frames are joined to the cross member towards the back of the vehicle body, and reach the widthwise center of the rear end panel. A guard block for dispersing and absorbing impact loads received by a bumper beam during rear impact at low speeds is disposed in the widthwise center of the rear end panel.
Thus, when an impact load acts on the rear of the vehicle body, the impact load is transferred by the left and right diagonal members to the left and right rear frames and the cross member, which disperse the load.
However, since the left and right diagonal members extend in a straight line, when an impact load acts on the rear of the vehicle body, it is difficult for the diagonal members to satisfactorily deform to absorb the impact load.
Furthermore, in another known example of a rear frame structure of a vehicle, the rigidity of rear side frames against rear impact is increased, as is disclosed in JP 2006-88886 A. In this rear frame structure, the rigidity of the rear side frames is increased by providing a gusset over the rear ends of the rear side frames, and by providing a partitioning wall to the cross section of the rear side frames, corresponding to the position where the gusset is placed.
In the rear frame structure in JP 2006-88886 A, when a load acts on the rear side frames from the rear of the vehicle body as a result of a rear collision, the gusset functions to prevent the rear side frames from buckling.
Within the rear ends of the rear side frames, the bottom parts begin to buckle. When this buckling causes the bottom parts to reach the partitioning wall, the front-to-back region of the partitioning wall moves upward. As a result, the orientation of the load acting on the bending parts of the rear side frames is changed to a direction parallel to that of the bending parts, and the rigidity of the rear side frames is increased.
Hybrid automobiles include a large battery for supplying a voltage to the motor. This large battery is usually installed in the rear of the vehicle body.
In view of this, when a load acts on the rear side frames from the rear of the vehicle body, it is preferable to further increase the rigidity of the rear frame structure of the vehicle in order to protect the large battery.